The present invention relates to electronic circuits for sensing electrical current in a load conductor and relates more particularly to such circuits for use in a ground fault interrupting circuit breaker.
In a power system which is referenced to ground potential, all the current from the load is intended to return to the location of the breaker via a neutral wire. If any portion of the return current is absent at the breaker, then there must be a ground fault present which is permitting some other return path to be established to ground. A ground fault interrupting circuit breaker senses such a condition and interrupts the circuit in response thereto.
Ground faults in a ground-referenced power system are typically detected by monitoring the differential current in conductors feeding a load circuit. If such a system is operating normally, all current transmitted to the load will return on the conductors provided. This will result in a net zero current in all conductors at any instant in time if all conductor currents are summed, taking proper polarity into account. Any difference current is presumed to be returning to the source via a spurious ground path representing a "ground fault".
Differential current can be detected very simply by passing all load circuit conductors through a current sensor in the form of a current transformer. If all currents sum to zero, there will be no output from the current transformer secondary winding. Any differential current will produce an output that can be measured for ground fault detection. To test such a system, it is desirable to pass a test current with a known amplitude through the current transformer to produce an output in the secondary winding that should cause a prescribed response from the detection system. With this approach, all components of the system are tested under realistic conditions, leading to a high degree of confidence in the results.
Variations in both the load equipment and in the manner in which the loads are connected in a power system make it desirable that the sensitivity of the breaker be adjustable to suit its particular application. In addition, it is necessary to be able to readily test the operation of the breaker to assure that it is still functional and that the sensitivity is indeed as intended. A sure test of the entire circuit is to generate in the current transformer of the sensing circuit itself a current which somewhat exceeds the current threshold set by the sensitivity adjustment. However, when the sensitivity is adjusted to a different value the test current must also be changed.
There is a need for a current sensing relay circuit which has an adjustable sensitivity and which can also be readily and reliably tested.